It is well known to use metallic pigments, such as aluminum flakes, or inorganic pigments in coating compositions to provide metallic glamour or a desired color, chiefly on exterior panels of automobiles, motorcycles, etc. The use of these pigments in solvent based coating compositions results in relatively few problems. In recent years, use of water based coating compositions is strongly required from the viewpoint of prevention of environmental pollution.
However, when being utilized in water based coating compositions, the pigments, particularly aluminum flakes, react with water and any acid components present in such coating compositions. Then, the pigments deteriorate and can cause the generation of hydrogen gas. Furthermore, the finish appearances resulting from such coating compositions have a reduced brightness and glamour.
In order to avoid such problem, phosphated random polymers have been added to water based coating compositions to protect the surface of the pigments based on the mechanism that the phosphated portion of the polymers provides passivation of the pigments so as to inhibit the reaction caused by direct contact of water with the pigments.
In this regard, Du Pont has disclosed a series of phosphated polymers which are added to pigment-containing coating compositions to protect the surface of pigments. Concerning the phosphated polymers disclosed by Du Pont Company, a phosphate group is provided by the reaction of glycidyl(meth)acrylate polyacrylate with phosphoric acid ester. For example, U.S. Pat. No. 5,104,922, issued on Apr. 14, 1992, has disclosed a water based metallic coating material comprising a phosphated linear polymer. U.S. Pat. No. 5,530,070, issued Jun. 25, 1996, has disclosed a water based metallic coating material comprising a phosphated graft copolymer having macromonomer side chains.
Kansai Paint Company has disclosed a series of phosphated polymers which are added to pigment-containing coating compositions to protect the surface of pigments. For the phospated polymers disclosed by Kansai Paint Company, a phosphate group is provided by phosphated acrylic monomer, instead of by the reaction of glycidyl(meth)acrylate polyacrylate with phosphoric acid ester. For example, U.S. Pat. No. 6,099,968, issued on Aug. 8, 2000, has disclosed a water based metallic coating material comprising a phosphated polymer obtained by copolymerizing: styrene, alkyl(meth)acrylate, phosphoric ester group-containing polymerizable unsaturated monomer, carboxyl-containing polymerizable unsaturated monomer, and hydroxyl-containing polymerizable unsaturated monomer. U.S. Pat. No. 6,617,409, issued on Sep. 9, 2003, has disclosed a water based metallic coating material comprising a phosphated polymer obtained by copolymerizing: a polymerizable monomer having a phosphate group, a polymerizable monomer having a phosphoric acid group, and a (meth)acrylate based polymerizable monomer having two tertiary alkyl groups in a molecular.
However, the novel phosphate group-containing resin provided by the present invention allows pigments to have improved dispersivity in water based coating compositions. In addition, the novel phosphate group-containing resin provided by the present invention offers an improved protection to pigments against the reaction with water, thereby improving the stability of coating compositions formulated with pigments.